Wind turbines today incorporate control systems whose objective is to maximise energy production by a solution of compromise, which guarantees that the loads associated to the wind are maintained within a range that can be assumed by the component design. It is for this reason that the control over the torque demanded of the generator to operate at variable speed and the control of the blade pitch angle have become conventional control systems.
On the one hand, the control of the blade pitch angle is controlled by actuators disposed in the root of each blade, which makes the blade rotate around its longitudinal axis. Said actuation manages to vary the aerodynamic behaviour of the blade modifying the pitch of the blade to maintain the rotational speed at approximately the rated rotational speed once the rated torque has been reached.
On the other hand, the control system over the demanded torque modulates said demanded torque to the generator from the converter in accordance with the generator speed.
In this way, variable-speed wind turbines operate by maximizing the aerodynamic performance in a low-wind basis thanks to its capacity for working at variable rotor rotational speed, where their rotational speed is controlled by modulating the torque in the electrical generator in a determined medium and high-wind regime limit the rotational speed of the rotor in the event of gusts thanks to the control of the blade pitch angle, managing to limit the power generated in a value substantially equal to the rated power.
Generally, the wind turbine regulation loops calculate orders in working cycles whose calculation period is less than a second, with the aim of achieving great response agility. Thus, document U.S. Pat. No. 6,420,795B2 presents a torque control system such as those described above.
Said control system may involve an energy loss with respect to that which may be produced on many occasions due to the fact that said regulation loops incorporate limiters which, in each calculation step, prevent the power or the torque from exceeding their respective maximum value at any time.
To resolve said problem, there are proposals which include two regulation types: a regulation such as the conventional one, with the different speed control bands typical of variable-speed wind turbines, and an alternative regulation used in certain periods of time, wherein a higher value than the rated value of the conventional regulation loop is used as power limit, as a channel for the compensation of previous energy losses which may be due to situations wherein there was no wind available to reach the rated power.
Among the above we have document EP2336558A2 relating to control method of wind turbines comprising control means for a standard regulation tracking a power vs. generator speed curve of the generator, comprising a first sub-nominal zone wherein the generator speed is kept constant at its coupling value, a second sub-nominal zone wherein it allows that both the speed of the generator and the power increase or decrease in line with the wind speed, a third sub-nominal zone wherein the wind speed is kept constant at its nominal value and a nominal zone wherein the power is kept constant at its nominal value, where during certain periods of time an alternative regulation is used at a power higher than that determined by the standard regulation to optimise the energy production compensating for previous losses.
Patent application US2006273595A1 is also known relating to a system and method to operate a wind farm under high wind conditions, where the method performs an intermittent increase in the rated power output of the wind turbine based on an estimate of a set of operating parameters with respect to the design conditions of said operation parameters.
Likewise, in wind farm control loops which send rated power limitation orders, to stop below the order values, higher rated limits are applied to fast-acting or instantaneous regulation loops, with calculation cycles for the torque orders in the range of 10-100 ms and of the pitch orders in the range of 100-200 ms and the regulation loops are performed based on instantaneous measurements, so that the production can be extra-limited.